The human brain is an exceedingly complex processing system, which integrates continual streams of incoming sensory input data with stored memories, uses the input data and memories in complex decision processes at both conscious and unconscious levels, and on the basis of these processes generates observable behaviors by activation of its motor or movement control pathways and the muscles which these innervate.
In certain cases of traumatic injury or neurological disease, however, the brain is partially isolated from the periphery. Input data from certain senses are thus lost, at least for a portion of the body, as are many voluntary movements. Spinal cord injury is a well known example. With spinal cord injury, the pathways that link higher brain regions with the spinal cord and that are used for control of voluntary movements may be functionally transected at the site of injury. As a result, the patient is paralyzed, and (s)he can no longer voluntarily activate muscles that are innervated by regions of the spinal cord below the level of the injury. Despite the injury to their long fibers, however, many of the cells in these higher brain regions that control voluntary movement will survive and can still be activated voluntarily to generate electric signals for controlling voluntary movement. By recording directly from these cells with implantable devices (e.g., electrode arrays), signals generated by the cells may be "exteriorized" and used for the control of external prostheses, such as an assist robot or an artificial arm, or functional electrical stimulation paralyzed muscles.
Another example of such loss occurs in cases of amyotrophic lateral sclerosis (Lou Gebrig's Disease), in which the motor neurons which control muscles, as well as some of the brain cells that control these motor neurons, degenerate. In advanced stages of this disease, the patient may have completely intact senses and thought processes, but is "locked in", so that neither movements nor behavioral expressions of any kind can be made. Providing these patients with some way of communicating with the external world would greatly enhance their quality of life.
In sum, there is a need to develop a system for monitoring and processing the electrical signals from neurons within the central nervous system, so that the brain's electrical activity may be "exteriorized" and used for the voluntary control of external prostheses or assist devices. In this way, damaged pathways are circumvented and some control of the environment can be restored. Because the electrical fields of small groups of neurons drop off rapidly with distance from the cells, this system should include surgically implanted "tiny" electrodes or sensors, which can be placed in close proximity to the cells that generate command signals for voluntary movement.
Earlier attempts to utilize signals recorded directly from neurons for the express purpose of controlling external prostheses have, however, encountered a number of technical difficulties. A major problem is how to obtain stable electrical signals of sufficient amplitude for real-time control of an external device. Two previous approaches have been used, but neither is successful in this regard.
In the first approach, microelectrodes with small tips (&lt;300 .mu.m sq. surface area) have been used, which are positioned to within 10-100 .mu.m of a single neuron, thus isolating its action potentials or "spikes" from that of other, more distant cells. In some cases, two or three adjacent neurons are recorded from simultaneously. In such cases, electronic devices are used to discriminate between the spikes of the individual cells and to sort their "spike trains" into distinctly recognizable signals. One problem with this approach, however, is that the effective "isolation" of the spikes of only one to a few neurons requires that the recording electrode be positioned in close proximity to the neurons. Thus, to obtain such records from a sufficient number of movement-related brain cells, scores of electrodes should be implanted in the hope that a few of them will be in just the right position to record signals from one or only a few movement controlling cells. Given the required proximity of the electrode and the cells, there is a high probability, however, that small movements of the former with respect to the latter will result in signal loss either because the electrode moves slightly away from the cells of interest, or closer to them, resulting in cellular injury. With blood pressure induced pulsations of the brain within the skull, such relative movement is not only possible but very likely.
In recent years, small, multichannel, micromachined (integrated circuit) electrodes have been developed for use in neural recording. Given sufficient recording channel density, these electrodes promised a partial solution to the electrode/tissue movement problem described above. If the signal was lost from one channel by electrode movement, there was hope that it might be "picked" up by an adjacent channel, which moved closer to the active neuron at the same time that the previous one moved away. However, problems have been encountered with these electrodes as well. For reasons that are not entirely clear, neural signals are lost from these electrodes over time, due apparently to the formation of polarization potentials at dissimilar metal junctions along the recording channel, or the ensheathment and thus biological insulation of the electrode by glial cells.
A second approach is to use electrodes with larger exposed recording surfaces (in the range of 0.5 to 1.5 mm sq. surface area). These low impedance electrodes have lower noise characteristics than those with smaller tips, and can reliably record the activity of hundreds to thousands of neurons at greater distances than can the latter. Indeed, low level electroencephalographic (EEG) or field potentials can even be recorded from the surface of the scalp. This approach thus can avoid the difficulty of different signal output levels caused by small movements between the electrodes and the selected cells encountered in the first approach. The use of the signals recorded in the second approach presents, however, a major problem for prosthesis control. In such recordings, the desired control signals may be of very low amplitude and may be "buried" within, or confounded by, EEG potentials from neurons that are not involved in voluntary motor processes. Thus, averaging must be used over many movement attempts to extract a usable signal. For this reason, this approach is less than desirable and perhaps not useful for real-time neural control of an external device.
Another problem, which occurs regardless of the electrode type used, is that neural signals may change over time for a variety of reasons: e.g., (a) naturally occurring cell death, which occurs randomly throughout the brain in adults; or (b) learning processes, which may, over time, alter the quantitative relationship between a neuron's activity and the external parts of the body to which it contributes voluntary control.